In wireless communication networks, increasing high data rate availability at the cell edge is a key aspect in improving overall network capacity and the individual user experience. In many networks, the cell edge performance is not limited by the available signal power, or the user equipment (UE) receiver sensitivity. Instead, the main limitation comes from the interference from neighboring cell base stations.
A typical measure of the signal quality experienced by a given UE is the ratio of the own-cell (desired) signal power to the total other-cell interference plus noise power. That ratio is also known as the “geometry factor.” A given cell in the network functions as the “serving” cell for the UE, and the other-cell interference arises from one or more neighboring base stations operating on the same carrier frequency as the serving cell base station. The (white) noise is typically due to the UE receiver noise, but may also include leakage from neighboring carriers, other systems, etc.
Advanced receiver structures may be used to improve the effective geometry factor. For example, the Third Generation Partnership Project (3GPP) specifications for High Speed Downlink Packet Access (HSDPA) define an enhanced receiver capability referred to as “Type 3i.” Type 3i receiver features include a linear dual-antenna receiver that utilizes its spatial degree of freedom to steer a spatial null towards the dominant interfering neighboring cell signal. The interfering signal is treated statistically, without invoking assumptions about its internal signal structure or data contents. In scenarios where one neighbor cell dominates, attractive gains may be available from such suppression, at least under certain channel conditions.
While the linear Type 3i receiver may offer gains in some situations, its efficiency is limited by the constraints intrinsic to linear receivers in general. The spatial degrees of freedom are limited to the number of antennas, i.e., a dual-antenna receiver can suppress one dominant interferer while receiving a desired signal from the own cell. At the same time, by spending the available degree of freedom on neighbor-cell suppression, the spatial suppression feature is no longer available for mitigating multipath-induced interference.
Unfortunately, in CDMA-based systems, like WCDMA/HSPA, inter-symbol interference (ISI) and multi-user interference (MUI) caused by dispersive channels represent major performance-degrading elements. In that regard, reduction in the ISI suppression capability is an undesirable side effect of linear neighbor-cell suppression. Further, when more than one cell contributes significant interference, linear receivers cannot suppress them simultaneously.
A well-known alternative to linear interference suppression is interference cancellation (IC). In that case, the interfering signal is no longer considered statistically, but its internal structure is explicitly utilized. Interference cancellation involves estimating the contents (partial or full) of the interferer, regenerating the interferer's signal contribution, and explicitly subtracting the resulting interference estimate from the received signal prior to demodulating and decoding the received signal.
Typically, HSDPA data makes up most of the transmitted signal power from a HSDPA base station (BS). Essential information required for performing HS data demodulation and decoding is transmitted to individual UEs via the High Speed Shared Control Channel (HS-SCCH) channel. For example, the control signaling comprising a given HS-SCCH transmission to a particular UE includes a first part that tells the UE what signal structure will be used to make a corresponding data transmission to the UE on a High Speed Physical Downlink Shared Channel (HS-PDSCH), and a second part that provides the UE with transport format information for that corresponding HS-PDSCH data transmission.
In particular, the first part of an HS-SCCH transmission includes parameters like modulation (QAM) order, the (channel) code allocation used for the corresponding HS-PDSCH data transmission, and the MIMO rank of that transmission. This first part (“Part 1”) may be broadly regarded as containing “demodulation information” needed by the targeted UE for properly demodulating the HS-PDSCH transmission targeted to that UE.
In complementary fashion, the second part (“Part 2”) of the HS-SCCH transmission includes transport format information, required by the UE for proper decoding of the HS-PDSCH transmission. For example, Part 2 includes information like the transport block size, code rate, and HARQ process info, etc. The first and second parts of an HS-SCCH transmission are individually coded and a common CRC is included in Part 2.
While the interested reader may refer to Section 4.6 of the 3GPP Technical Specification entitled “TS 25.212” for exhaustive details, here it suffices to explain that Part 1 of an HS-SCCH transmission is “masked” with the identity (ID) of the specific UE targeted by the transmission, according to a known masking function. Because the conventional behavior of UEs is to “look” for HS-SCCH transmissions individually targeted to them, HS-SCCH processing traditionally involves each UE using its own (known) ID to blindly detect Part 1 transmissions masked with the UE's ID. Once such a transmission is detected, the UE uses the decoded Part 1 and Part 2 contents to determine the signal structure and transport format of the correspondingly targeted HS-PDSCH data transmission, for proper reception, demodulation, and decoding of that corresponding data transmission.
Notably, Part 2 of an HS-SCCH transmission is not masked, except that Part 2 includes or is otherwise appended with a Cycle Redundancy Check (CRC) value that is masked with the UE ID. The CRC is computed over the (unmasked) contents of Part 1 and Part 2 at the transmitter. Therefore, in conventional processing, the UE uses its own known identity to unmask the CRC of an HS-SCCH transmission that has been determined from the Part 1 masking as being targeted to it, and then confirms Part 1 and Part 2 decoding using the unmasked CRC.
Coordinated Multi-Point (CoMP) transmission schemes, either centralized or distributed, represent one approach to reducing interference in multi-cell/multi-user wireless communication networks. According to CoMP operation, transmissions are jointly coordinated across multiple base stations, to reduce other-cell interference. However, in older (legacy) networks that do not implement CoMP, neighbor-cell transmissions are not “cooperative” and a given UE operating in its serving cell has no signal structure (Modulation and Coding Scheme or MCS) information available for users other than itself. That is, HS-SCCH transmissions to a first UE identify the MCS to be used for data transmissions on the HS-PDSCH to the first UE, but that first UE does not have signal structure information for potentially interfering HS-PDSCH data transmissions to other users, e.g., other UEs in the same or neighboring cells receiving HS-PDSCH transmissions that interfere with the first UE's reception of its targeted HS-PDSCH transmission.
While the modulation parameters (QAM mode, code allocation), required for pre-decoding IC, may be detected blindly, the detection quality may not be sufficient for robust operation. Moreover, blind detection of the transport format of interfering HS-PDSCH transmissions is not feasible and that prevents the use of better-performing post-decoding IC. Therefore, efficient neighbor-cell IC in HSDPA requires access to the interfering user's HS-SCCH information, implying the ability to infer its UE ID.
There are known approaches aimed at obtaining the IDs of UEs that are active in or around a given UE, but such approaches presume knowledge of the Part 1 transmission contents targeting such other users. Heretofore, there has been no high-performance, generally applicable mechanism for identifying the IDs of unknown UEs that are active in or around a given UE, which limits the opportunities for improving HSDPA performance in legacy (non-CoMP) systems.